Dopamine challenge reveals neuroadaptive changes in marijuana abusers

The current watershed in legal status and rising use of marijuana can be traced to a Cal-ifornia ballot initiative (Prop. 215, its legal successor SB420), that enabled widespread access to smokeable or edible forms of marijuana for self-reported medical conditions. Circumventing the Food and Drug Adminis-tration (FDA) drug approval process, the movement in California was replicated by ballot or legislative initiatives in 23 states and the District of Columbia, and culminated in the legalization of marijuana in 2012 by Washington state and Colorado. The shifting status of marijuana reflects a change in public perception and belief that marijuana is harm-less. Marijuana use in the population over age 12 is escalating; 60% of 12th graders do not perceive marijuana as harmful, and daily or nearly daily use has risen dramatically in this cohort (1, 2). Paradoxically, public perception of marijuana as a safe drug is rising simulta-neously with accumulating evidence that frequent marijuana use is associated with adverse consequences, especially among youth (3). In PNAS, Volkow et al. register compelling new observations that marijuana abusers manifest adaptive behavioral, physiological, and biological responses, which conceivably contribute to marijuana addiction and com-promised function (4). In response to a dopamine challenge (methylphenidate) and compared with non-using controls, marijuana abusers self-reported blunted reward (less “high”) and heightened negative responses (anxiety and restlessness), which were associated with attenuated dopamine responses in brain and cardiovascular responses.

By integrating behavioral and brain-imaging measures following a dopamine challenge (methylphenidate) in marijuana abusers, Volkow et al. (4) add a new di-mension to clarifying the impact of long-term marijuana use on brain dopamine response. Methylphenidate, a surrogate for dopamine, elevates extracellular levels of dopamine (and norepinephrine) by blocking the dopamine transporter (DAT) in dopa-mine-expressing neurons. As the DAT sequesters dopamine in dopamine-releasing neurons, the blockade raises extracellular dopamine levels in dopamine-rich brain regions. The rapid rise in dopamine triggers self-reports of a “high.” Marijuana abusers self-reported blunted measures of “high,” drug effects, increased anxiety, and rest-lessness. The magnitude and peak behavioral effects of methylphenidate were more robust in controls than marijuana abusers. Cardiovascular responses (diastolic blood pressure, pulse rate) were also attenuated in the abusers. Significantly, the younger marijuana use was initiated, the higher the scores for negative emotionality. These findings reinforce the accumulating evidence that earlier age of initiation of mar-ijuana abuse is associated with worse out-comes (3, 16). Collectively this phase of the study suggests that brain dopaminergic, pos-sibly noradrenergic systems, are significan-tly modified in long-term, heavy marijuana abusers. These changes conceivably contribute to reduced rewarding effects, emotion-ality and motivation, increased propensity for addiction, with early initiators being more vulnerable.

D2/D3 dopamine receptors are critical mediators of the initial responses to drugs of abuse. PET imaging of brain revealed a more complex pattern of change in dopamine signaling than previously reported for other specific drugs of abuse. D2/D3 dopamine receptor availability, measured with the D2/D3 receptor antagonist [11C]raclopride, was not reduced in marijuana abusers, in contrast to reduced dopamine receptor availability observed in subjects with other specific substance use disorders (11–14).

This conclusion remains tentative, as the age of the marijuana-abusing cohort was considerably younger than drug-abusing subjects previously interrogated for D2 dopamine receptor availability.

[11C]Raclopride can also serve as an in-direct measure of dopamine production, release, and extracellular levels (17). Reduced [11C]raclopride binding-site availability is detectable following administration of a psy-chostimulant (e.g., methylphenidate or amphetamine), which elevates the extracellular dopamine by blocking transport or promoting its release from neurons. The dopamine surge competes with [11C]raclopride for binding to the D2/D3 receptor, with [11C]raclopride displacement proportional to extracellular dopamine. In marijuana abusers, diminished dopamine responses were observed in the ventral striatum compared with controls, and were inversely correlated with addiction severity and craving. The attenuated responses to methylphe-nidate are consistent with decreased brain reactivity to dopamine stimulation in marijuana abusers, which conceivably contributes to the increase in stress responses, irritability, and addictive behaviors. Thus, marijuana joins the roster of other abusable drugs in promoting blunted dopaminergic responses in a brain region implicated in drug reward, but deviates from other drugs in that it apparently does not promote a decline in D2/D3 receptor availability.

The study yielded several unanticipated discoveries. Marijuana abusers displayed enhanced dopamine release in the substantia nigra/subthalamic nucleus, which correlated with marijuana and tobacco craving, as well as addiction severity. Because this brain re-gion has relatively high densities of the D3 receptor, this preliminary finding reinforces the need to expand PET imaging to multiple, discrete brain regions, with higher-resolution cameras, and to enlist other probes capable of selective monitoring of each of five dopamine receptor subtypes. Another surprising obser-vationwasthedecreaseindistributionvolume in the cerebellum by methylphenidate in con-trols, but not in marijuana abusers, another manifestationofabluntedresponse.Thisbrain region characteristically is used as a reference region to normalize for nonspecific binding (“baseline”) of PET imaging probes if comparing group differences, possibly resulting in overestimates of the methylphenidate re-sponse in other brain regions of marijuana abusers. This finding, which may reflect vas-cular changes engendered by marijuana, highlights the necessity of heightened scrutiny ofthe cerebellum asa“neutral”baseline region for dopamine receptor monitoring in group comparisons.

Collectively, abnormal behavioral responses to a methylphenidate challenge implicate dopamine signaling adaptation in mari-juana abusers. Even though a decrease in striatal D2 receptor density does not ac-count for the responses, other components of the synapse (e.g., DAT, dopamine syn-thetic capacity, the dopamine signaling cascade, events downstream of dopamine receptors) conceivably contribute to mani-festations of blunted subjective responses.

Future Multidisciplinary Research

The current research (4), providing strong evidence that marijuana abuse is associated with blunted dopamine responses and re-ward, is a major contribution to a growing body of evidence that heavy marijuana use is associated with brain changes that could be detrimental to normal brain function. Numerous other brain-imaging studies have been conducted in heavy adult marijuana users (e.g., ref. 18), with reported changes in brain morphology and density, defor-mation of specific structures, altered con-nectome (e.g., hippocampus), and function.

The current research, which integrates be-havioral and physiological changes within the context of a specific neurochemical substrate, dopamine, provides important leads for in-tegrating with other changes gleaned from MRI technologies. Intriguingly, evidence that dopamine receptor signaling can affect ex-pression of genes encoding axonal guidance molecules that are critical for brain devel-opment and neuroadaptation (19) may pro-vide a link between drug-induced receptor activity and gross and discrete altered mor-phology and circuitry characteristic of the drug-adapted brain.

There remains a compelling need for prospective, integrated longitudinal research in this field, especially in adolescent mari-juana users, as the impact of marijuana on the developing brain is more robust with early age of initiation (3, 16). Imaging studies are predominantly snapshots in time, relying on self-reports of marijuana use, dose, and frequency, with subjects of varying ages, group sizes, differing imaging tech-niques, and other variables that confound meta-analyses or integration of data from different sites to expand study power. A critical longitudinal study showing a signifi-cant IQ decline in early marijuana users is a prime example of the direction in which the field should be going, but with co-ordinated brain-imaging approaches (20).

Preclinical studies can circumvent the limitations of some clinical metrics, and es-tablish causality for specific changes that are not feasible to measure in humans. Yet the divergence of the human brain anatomically and functionally limits unfettered extrapola-tion from animals to humans. Large-scale, multicenter prospective longitudinal human research starting before initiation of drug use and extending for three decades of life is needed to further pursue causal relation-ships of marijuana and adverse consequences reported in numerous shorter-term studies. Research design could include: (i) brain im-aging to document occurrence of, resolution, or persistence of structural, circuitry, vascu-lar, and associated and neuropsychological decrements; (ii) neurocognitive function; (iii) behavioral, emotional assessment; (iv) neural, cognitive, epigenetic, proteomic, and affec-tive markers; and (v) preclinical, relevant parallel studies.

In view of the growing public health con-cerns of escalating high-dose, high-frequency marijuana use, early age of initiation and daily use, high prevalence of marijuana addiction, rising treatment needs, the void of effective treatment, high rates of relapse, association with psychosis and IQ reduc-tion, a rising tide of emergency room epi-sodes, and vehicular deaths, constitute compelling reasons to expand marijuana research and to clarify its underlying biology and treatment targets/strategies. Longitudinal studies that begin before initiation of use, and that integrate brain imaging with behavioral, cognitive, and other parameters, will facilitate shaping of public perception and public policy with more informed scientific evidence.